This invention relates to a method and apparatus of automatically setting PlD (proportional plus integral plus derivation) constants used for process control and more particularly to a method and apparatus for auto-tuning (automatic adjustment of constants) PID control suitable for automatically setting, on the basis of a control response from a process, PID constants which can rapidly settle the control response in accordance with a change in set point and a disturbance applied.
In PID control systems, the setting of PID constants has hitherto been done on the basis of a "rule of thumb" obtained from experience of skilled control engineers. In such a manual setting, however, the engineer is required to conduct tuning of PID constants each time start-up of a process is initiated or a change is introduced into process characteristics. With a view of eliminating such troublesome labor, a PID controller has been proposed which possesses the function of auto-tuning PID constants.
The control response characteristic due to the change in set point differs from that due to the applied disturbance and the PID constants for the former factor generally differ from those for the latter factor. When the same PID constants are used for the set-point change and disturbance, it is difficult to perform control for achieving control characteristics effective for both of the set-point change and disturbance, that is, such control as for designating, for example, a damping ratio of 10% and an overshoot amount of 5%. JP-A-62-241003 discloses a prior art auto-tuning PID controller based on fuzzy reasoning which uses a single knowledge base and characteristic values to perform tuning. Disadvantageously, this prior art controller sets PID constants effective for only one of a set-point change and a disturbance and can not settle control response effective for both of the set-point change and the disturbance.
U.S. Pat. application (PID Controller System) Ser. No. 177,428 assigned to the same assignee and filed on Apr. 1, 1988 is also concerned with auto-tuning of PID constants based on fuzzy reasoning.
According to the disclosures of JP-A-61-290505 and JP-A-62-9404, PID constants effective for only one of the set-point change and the disturbance are set and when a disturbance or a change in set point occurs, the PID constants are corrected by a compensation calculation means to PID constants equivalent to an control status which is optimized for suppression of a change due to the occurring factor.